飞秒激光作用下光整流晶体的损伤阈值分析

Analysis on damage thresholds of optical rectification crystals under femtosecond lasers

光整流效应是产生太赫兹波的有效途径之一,而产生高功率太赫兹波则需要采用高强度的飞秒激光激励光整流晶体,但激光强度过大会造成晶体损伤,从而严重影响高功率太赫兹波的输出。因此,对光整流晶体的损伤阈值进行研究具有重要的理论意义与实用价值。通过对飞秒激光与非线性光整流晶体材料相互作用物理过程进行分析,建立了飞秒激光作用下光整流晶体损伤阈值的预估模型,比较分析了铌酸锂、碲化锌和硒化锌三种晶体材料的损伤阈值随飞秒激光脉宽的变化规律。结果表明,当飞秒激光的脉宽相对较小时,光致电离在整个作用过程中占主导地位,随着脉宽的增大,雪崩电离逐渐起主要作用;雪崩电离速率和光致电离速率均与晶体的禁带宽度密切有关,光整流晶体材料禁带宽度越大,晶体材料的损伤阈值越高;铌酸锂晶体的损伤阈值明显高于碲化锌和硒化锌晶体,更适合用于高功率太赫兹波的产生。

Optical rectification effect is one of the effective ways to generate terahertz （THz） wave, and the generation of high-power THz wave requires femtosecond laser with high intensity to excite optical rectification crystals. However, the crystal will be damaged if the intensity of femtosecond laser is too high, seriously affecting the output of high-power THz wave. Therefore, it is of great importance to study the damage thresholds of optical rectification crystals for both theoretically and practically. In this paper, the interaction mechanism between the femtosecond laser and the nonlinear optical rectification crystals was analyzed, and the prediction model of damage thresholds of optical rectification crystals under femtosecond laser was built up. Then on this basis, the variation of damage thresholds of LiNbO3,ZnTe and ZnSe crystals with the pulse duration of femtosecond laser was compared and analyzed. The results show that, the photoionization dominates in the whole process when the pulse duration of femtosecond laser is relatively short, while the avalanche ionization gradually plays an important role with the increase of pulse duration. Avalanche ionization rate and photoionization rate are related closely with the forbidden band width of the crystal. Consequently, the greater the forbidden band widths of optical rectification crystals, the higher the damage thresholds are. The damage thresholds of LiNbO3 are higher than those of ZnTe and ZnSe crystals, which is more suitable for the generation of high-power THz wave.